Fluidization of solid particles by gas is a well-known phenomenon. When a gas is passed upwards to a bed of fine solid particles, the state of the bed depends upon the gas flow rate as follows: At low flow rate gas merely percolates through the voids between the stationary particles and the bed is called fixed bed. However, with an increase in the gas flow rate, particles move apart forming expanded bed. With a further increase in the gas flow rate, a point is reached when all the particles are just suspended in the upward flowing gas and such a bed is referred to as an incipiently fluidized bed or a bed at minimum fluidization. When gas flow rate is increased beyond minimum fluidization, large instabilities in bubbling and channeling of gas are observed. At this stage, the movement of particles becomes more vigorous but the bed does not expand much beyond its volume at minimum fluidization. Such a bed is referred to as bubbling fluidized bed and it consists of two distinct phases: a non-continuous bubble phase consisting mainly of gas with a very low concentration of solid particles carried over by the gas bubbles, and a continuous emulsion phase consisting of solid particles and gas (Ref. J. F. Davidson and D. Harrison in Fluidized Particles, Cambridge University Press, 1963; D. Kunii and O. Lavenspiel in Fluidization Engineering, John Wiley and Sons. Inc., New York, 1969).
Most commercial gas fluidized bed reactors for catalytic and non-catalytic reaction operate in the bubbling regime as bubbling fluidized beds. In the bubbling fluidized bed reactor, the upward motion of the gas bubbles cause enough mixing of solid particles in the emulsion phase, also called as the dense phase and hence the temperature is nearly uniform in the entire reactor. This effect of gas bubble is favourable. However, because of the low concentration of solid particles in the gas bubbles, there is little reaction within the bubbles. Moreover, the bubbles serve as channels for gases to bypass the solid particles and leave the reactor more or less unreacted for the catalytic reaction, when the solid particles are catalyst particles, and also for the non-catalytic reactions, when the solid particles are in particles in case of thermal non-catalytic reactions e.g. thermal cracking of naphtha or with sand or solid reactant in case of non-catalytic gas-solid reactions, e.g. reduction of metal oxides by H2 and/or CO, regeneration of coked catalyst by oxidative treatment.
A number of commercial catalytic and non-catalytic reactions are carried out in bubbling fluidized bed reactors. Art method for the operation of bubbling fluidized bed reactor for gas phase catalytic or non-catalytic reaction involve a fluidization of solid particles in the reactor by reacting gas(es) at a superficial velocity which is much higher than that requird for anticipant fluidization or minimum fluidization of the solid particles, the reacting gas in excess for incipient or minimum fluidization passes through the reactor in the form of gas bubbles. The advantages of bubbling fluidized bed reactors are liquid like flow of solid particles, rapid mixing of solid particles leading nearly isothermal conditions throughout reactor, possibility or circulating solids between two fluidized beds so that catalysts particles coked due to catalytic reaction in one reactor can be transported to second reactor or their regeneration by oxidative treatment, high rates of heat transfer between a fluidized bed and a heat exchanger immersed within fluidized bed. All these advantages make the operation of bubbling fluidized bed reactor simple, easy and reliable from process control point of view. However, disadvantages or limitations of bubbling fluidized bed reactors used earlier are also many. The main limitation of using bubbling fluidized bed reactor for a catalytic or non-catalytic reaction is the difficulty in describing the flow of reacting gas through the emulsion phase and bubble phase, with its large deviation from plug flow and bypassing of the solid particles by reacting gas through gas bubbles, resulting in an inefficient contacting between solid particles and reacting gas. This becomes particularly very serious when high conversion of reacting gas is required. Commercial scale operations involve high gas throughputs, requiring large bed diameters and gas velocities. Both these factors lead to vigorously bubbling beds with large size bubbles with their serious bypassing and poor gas-solid contacting. Under such condition a high conversion can be attained by keeping contact time long by increasing reactor height for a given operating gas velocity at a cost of increased capital, increased catalytic cost increasing power requirement for pumping the gas streams. However, high selectivity cannot be achieved by this as it can be achieved only under high gas-solid contact efficiency. Also because of the non-ideal flow of the gas in the bubbling fluidized bed reactors and complex nature of the exchange of gas between emulsion phase and bubble phase, it is difficult to predict the reactor performance and also to design or scale-up the reactor (Ref D. Kunii and O. Levenspiel in Fluidization Engineering, John Wiley & Sons, Inc. 1969; O. Levenspiel in Chemical Reaction Engineering, 2nd Edn. Wiley Eastern Ltd., Y. Ikeda in Fluidization on 85: Science and Technology, Ed. Kunii et al, Conference papers, 2nd China-Japan Symposium, Kunming China, Science Press, Beizing China, Elsevier, Arnst 1985 p. 1) and W. Yongan et al, Ibid. p. 11).
It is preferable in commercial practice to avoid the bypassing of the reacting gas which is in excess of what required for incipient or minimum fluidization, so that a major limitation or drawback of bubbling fluidized bed reactors could be eliminated. The following are used in the prior art to overcome the problem of bypassing of reacting gas through gas bubbles:    (a) Internals are inserted into the bed to hinder gas bubble growth and also to cut down size of bubbles. This reduces bypassing of reacting gas through gas bubbles but only to a small extent.    (b) A combination of bubbling fluidized bed and packed bed reactor, where the gas first passes through the fluidized bed section and then through the packed bed section, is used for achieving high conversion of reacting gases. This reactor system is however, complex and difficult to operate and control and yet there is a bypass of reacting gas through bubbles of the bubbling bed reactor.    (c) A use of fast fluidization with less fluidized solid particles is also suggested. As compared to bubbling fluidized bed, the fast or lean fluidized bed has some advantages, such as the gas-solid contact efficiency is higher and the gas flow is plug or piston flow. However, it has also a serious drawback such as the solid particles in the reactor becomes very dilute and the advantages of bubbling fluidized bed in commercial operation are lost (References: O. Levenspiel in Chemical Reaction Engineering, 2nd edition, Wiley Eastern Ltd., and Y. Ikeda in Fluidization 1985: Science and Technology, Ed. Kunii et. al., Conference papers, 2nd China-Japan Symposium Kunming, China, Science Press, Beijing, China, Elsevier, Amst., 1985).
If the bypass of reacting gas(es) through gas bubbles in bubbling fluidized bed reactor is eliminated or drastically reduced, and thereby gas-solid contacting efficiency is increased, this would be of great practical importance for carrying out a number of highly exothermic, highly endothermic catalytic or temperature sensitive non-catalytic reactions, using bubbling fluidized bed reactors. Hence, thee is a need for developing an improved method for the operation of bubbling fluidized bed reactor to achieve this goal.